A person is standing outdoors in the shade where the temperature is 17 °C. (a) What is the radiant energy absorbed per second by his head when it is covered with hair? The surface area of the hair (assumed to be flat) is 140 cm2 and its emissivity is 0.87. (b) What would be the radiant energy absorbed per second by the same person if he were bald and the emissivity of his head were 0.68?

Question

Physics

Junior Hood

12 May, 09:19

A person is standing outdoors in the shade where the temperature is 17 °C. (a) What is the radiant energy absorbed per second by his head when it is covered with hair? The surface area of the hair (assumed to be flat) is 140 cm2 and its emissivity is 0.87. (b) What would be the radiant energy absorbed per second by the same person if he were bald and the emissivity of his head were 0.68?

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Answers (1)

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Quinn Chen · Answer 1

a) 4.9W b) 3.82W

Explanation:

Stefan-Boltzmann law of radiation formulae:

Q/t (W) = sigma * e*A*T^4

Sigma = 5.67*10^-8 j/sm^2K^4 (Stefano Boltzmann constant)

e = emissivity

T = absolute temperature in kelvin and A = area in m^2

a) Q/t = 140/10000 (m^2) * 0.87 * 5.67 * 10^-8 * (290^4) = 4.9W

b) without hair the

Q/t = 140 / 10000 (m^2) * 0.68 * 5.67 * 10^-8 * (290^4)

= 3.82W